Abstract : We investigate the prevalence and learning impact of different types of off-task behavior in classrooms where students are using intelligent tutoring software. We find that within the classrooms studied, no other type of off-task behavior is associated nearly so strongly with reduced learning as -gaming the system-: behavior aimed at obtaining correct answers and advancing within the tutoring curriculum by systematically taking advantage of regularities in the software-s feedback and help. A student-s frequency of gaming the system correlates as strongly to post-test score as the student-s prior domain knowledge and general academic achievement. Controlling for prior domain knowledge, students who frequently game the system score substantially lower on a post-test than students who never game the system. Analysis of students who choose to game the system suggests that learned helplessness or performance orientation might be better accounts for why students choose this behavior than lack of interest in the material. This analysis will inform the future re-design of tutors to respond appropriately when students game the system.